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(CNN)Cecil the lion is dead, killed illegally in Zimbabwe, authorities allege, by a foreign hunter or hunters who paid about $55,000 for the privilege.

Cecil was part of an Oxford University research project and wore a GPS collar.

He was lured out of a national park with food, shot with a crossbow, tracked for 40 more hours, then finished off with a gun, said Johnny Rodrigues, head of the Zimbabwe Conservation Task Force.

The 13-year-old lion's head was reportedly cut off as a trophy, and his skin was taken as well. Rodrigues told CNN that the head and skin had been found and confiscated, and were being processed as evidence.

Cecil was killed July 6, Rodrigues said, allegedly by a Spaniard. But he said that as many as three hunters may have been involved, none of whom have been detained. The nationalities of those involved have not been confirmed.

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Hunters group says it respects ongoing investigation

The operator of the safari has been arrested, and a hearing has been set for August 6.

"The professional hunter and company he works for have been cooperative in the investigation," the association said. "ZPHGA reiterates it will not tolerate any illegal hunting or any unethical practices by any of its members and their staff."

It asked everyone to respect the ongoing investigation and said it would not comment further until the inquiry is complete.

Researchers at Oxford University expressed grief at the lion's death and at the news that, for one reason or another, he had wandered away from the protection of the national park in Zimbabwe.

"It's not many months ago that I watched Cecil with my hand on my heart as he strayed toward a hunting concession," said professor David Macdonald, founding director of Oxford's Wildlife Conservation Research Unit. "On that occasion he turned back into the protection of the park, but this time he made a fatal mistake and I feel deeply sad, personally."

Macdonald said it was important to realize that lions live in complicated societies. Research has shown, he said, that if one male is killed, "the resulting perturbation" can lead to the deaths of other males and to the deaths of his cubs.

Cecil is survived by about six lionesses with whom he mated regularly and about 24 cubs, Rodrigues said.